Method of controlling the orientation of fibers in a foam formed sheet

ABSTRACT

IN THE PROCESS OF FOAM FORMING FIBERS INTO SHEET MATERIAL, THE FIBERS ARE ORIENTED TO THE DEGREE DESIRED BY CONTROLLING THE RELATIVE MOTION BETWEEN FOAM SEGMENTS. FOR RANDOM ORIENTATION, OPERATION IN THE PLUG FLOW REGIME IS PREFERRED WHILE OPERATION IN THE YIELD FLOW REGIME RESULTS IN INCREASE FIBER ORIENTATION IN THE DIRECTION OF FLOW. THE DEGREE OF FIBER ALIGNMENT CAN BE CONTROLLED BY VARYING THE WETTED PERIMETER OF THE NOZZLE FROM WHICH THE FOAM EXISTS ONTO THE CARRIER WIRE.

' METHOD OF CONTROLLING THE ORIENTATION OF FIBERS IN A FOAM FORMED SHEET 4 Sheetg-Sheetl Filed Dec. 20, 1971 FIGQII CHUNG 3,837,999

. R; CHUNG I METHOD OF CONTROLLING THE ORIENTATION OF Sept. 24, 1014 mamas :uxrom momma sanm- H I Filed Dec. 20, 1971 4. Sheets-Sheet z Sept 4' 1974 R. CHUNG 3,837,999

METHOD OF CONTROLLING THEORIENTATION of FIBERS IN A FORMED SHEET Filed Dec. 20, 1971 -4Sbeets-Sheet 5 l I I U I United States Patent O 3,837,999 METHOD OF CONTROLLING THE ORIENTATION F FIBERS IN A FOAM FORMED SHEET Raymond Chung, Neenah, Wis., assignor to Kimberly- Clark Corporation, Neenah, Wis. Filed Dec. 20, 1971, Ser. No. 209,749 Int. Cl. D21h 3/02 US. Cl. 162-101 2 Claims ABSTRACT OF THE DISCLOSURE In the process of foam forming fibers into sheet material, the fibers are oriented to the degree desired by controlling the relative motion between foam segments. For random orientation, operation in the plug flow regime is preferred while operation in the yield flow regime results in increased fiber orientation in the direction of flow. The degree of fiber alignment can be controlled by varying the wetted perimeter of the nozzle from which the foam exits onto the carrier wire.

DESCRIPTION OF THE INVENTION My invention relates generally to processes for forming fibrous webs. More particularly, my invention concerns such Web forming processes that include the step of generating a foam within which the fibers are dispersed. Still more particularly, the process of my invention is directed to a method for controlling the orientation of the fibers within such a foam and thereby obtaining desired physical properties in the resulting web.

Processes for foam-forming fibrous webs have been known for many years. Generally, these processes involve dispersing the fibers in water, adding a surfactant or other foaming aid, and mixing in air to form an aqueous foam; this foam is then usually deposited on a moving screen or wire, and the bulk of the fluids is drained in much the same manner as in ordinary paper-making processes. Webs varying widely in thickness have been made for different applications such as medical dressings, carpet pads, insulation, wallboard, and various papers. Advantages of the foam-forming process over conventional paper-making methods include a product with better formation, viz. a more uniform distribution of fibers to give a less cloudy appearance; easier handling since the foam is relatively stiff, not as mobile as a water dispersion, and the fibers within the foam are prevented from agglomeration and entanglement; reduced water handling since consistencies of 1 to 3% and even higher can be used; and others.

In spite of the foregoing advantages, foam forming has not become widely used. One of several reasons for its lack of success, particularly with respect to the manufacture of sheets of paper with basis Weights in the range from 5 to 50 lbs/3000 ft. is that apparatus for foam forming of which I am aware offer no control of fiber orientation. The webs formed directly from the foam contain fibers in a high degree of randomness.

It is a primary object of my invention to provide a method for controlling the degree of fiber orientation and related physical properties of foam-formed fibrous webs.

Related to this objective it is further an object of my invention to achieve such fiber orientation control by regulation of shear stresses throughout the foam while it is fluid and in motion.

In accordance with the foregoing objects, a still further objective of my invention is to provide a method for producing a foam-formed fibrous web having any desired ratio of machine direction (MD) strength to cross-machine direction (CD) strength within a significant range.

Other objects and advantages of my invention will become apparent to those skilled in this art upon reference to the detailed disclosure and to the drawings in which,

FIG. 1 is a schematic diagram of an arrangement which can be used to carry out a process embodying my invention.

FIGS. 2-6 show examples of different nozzles which can be used in accordance with my invention to obtain desired shear action in the flowing foam; and

FIGS. 79 illustrate the operation of my invention to produce shear forces on fibers within the foam and resulting fiber orientation.

As was earlier mentioned, known methods and devices for generating and applying a layer of fibrous foam result in a sheet containing fibers in substantially random orientation. While it is not intended that my invention be limited to any particular theory, I believe that the relatively low shear forces applied to the generated foam allow the fibers going through these known processes and apparatus to remain in a generally undisturbed, random orientation which persists in the final sheet. The sheet, as a consequence of this randomness, has generally equal strength properties in both the machine direction (MD) and the cross direction (CD). For some applications, this type of sheet is desirable. For others, such as backing in masking tapes and papers with good runnability used in continuous printing presses, various degrees of greater MD strength compared to CD strength are preferred; the degree depends on the end use of the sheet.

I have found that the relative MD/ CD strength properties of a foam-formed fibrous sheet may be controlled within a substantial range by the selection of certain flow parameters. I believe that by controlling the flow of the fibrous foam before and, if desired, while the sheet is being formed, the shear forces applied to the foam may be regulated and, in turn, a desired degree of fiber orientation can be obtained.

While the operation of my invention is not to be limited thereto, I prefer that the fibers be substantially randomly oriented as produced by the foam generator or oriented in a degree less than what would be ultimately desired. If orientation is more than desired or in any mode deemed undesirable, the foam is preferably mixed and such orientation destroyed. In this manner my invention may be utilized to develop fiber orientation to the degree required for desired sheet strength properties within the range of the process capabilities.

By control of the shear forces while the foam is flowing, I have found that the desired degree of selective orientation of the fibers within the foam may be obtained. I prefer to use nozzles for applying the foam to a screen or to any other device for removing fluids. By reducing the nozzle outlet area, by increasing the nozzle length, or by any other means increasing the wetted surface to volume ratio in the nozzle as the foam is directed to the wires, the shear forces on the fibrous foam may be increased. By wetted surfaces I mean those external surfaces which contact the bulk of the moving foam. When the foam ceases to sustain the shear force applied to it, it begins to enter into the yield flow range defined as the state where relative movement takes place between layers of bubbles in the foam. At the beginning of yield flow, only the segments or layers of foam closest to the wetted surfaces exhibit relative motion with respect to each other. As shear increases, more and more bubble layers or segments move past the adjacent ones until finally relative movement exists between all layers.

[I believe that, since the fibers are wetted by the liquid, they are lodged in the bubble membranes, and, where the fiber length is great relative to the bubble size, these fibers span a plurality of bubble layers. Thus, as movement between bubble layers takes place, the fibers tend to become aligned or oriented in the direction of flow. The more the number of bubble layers move by each other, in general, the greater will be the number of highly oriented fibers and the more directional the sheets strength properties will be. This is in contrast to conventional papermaking techniques where the fibers are dispersed in water and move more freely tending to resist this alignment effect.

Referring now to the drawings, FIG. 1 illustrates schematically a foam-forming system of the type wherein my invention may be employed. Foam generator receives the foamable fluid feed generally indicated 11 and feeds the foam 13 through nozzle 12 to vertical forming screens 14 and 16. Screen 14 moves endlessly about a conventional roll system comprising breast roll 18a, stretch roll 18]), wire rolls 18c and 18a, guide roll 18d and couch roll 18f. Matching screen 16 is also supported by a roll system 'which consists of breast roll 19a, guide roll 1%, wire roll 19c, stretch roll 19d, and wire roll 19a. Both screens move over knee roll 19 In the arrangement shown, screen 14 rotates generally counter-clockwise while screen 16 moves in the opposite direction. In the figure the distance of separation between the screens is exaggerated for purposes of clarity. However they actually move in very close proximity to each other with the exact distance being adjustable and dependent upon the thickness or basis weight desired in the finished sheet. As the foam moves along with wires 14 and 16, it is subjected to pressure which tends to increase uniformity of distribution in the transverse direction. This pressure also benefits drainage by causing fluids to pass through screens 14 and 16. In addition these fluids are removed in part by foils 23 as well as by suction boxes 24 and 25 adjacent screen 14 and by foils and/or blades 26 adjacent screen 16. At this point the foam web is formed, and further operations comprise drying and finishing which may be carried out in conventional steps used for paper in general. In the illustrated embodiment, the foamed stock 13 travels with wires 14 and 16 to roll 19f where a web 32 is substantially formed; suction boxes 29 and 30 serve to remove excess fluid and to retain the web on lower wire 14 and pan 28 collects excess fluid. Felt travels endlessly about pickup roll 21a, pressure roll 21b, stretch roll 21c, felt roll 21d, guide roll 21e, and felt rolls 21 and 21g. Transfer of web 32 from wire 14 to felt 20 occurs at the nip formed by rolls 18 and 21a. The web 32 is subsequently transferred from pressure roll 21b to Yankee dryer 36.

Roll 38 is the dancer roll which is movably supported in a conventional manner. From the dryer 36, web 32 is wound upon itself or subjected to other customary finishing operations. The fluid removed from foam 13 and web 32 is collected as generally indicated at tank 31 and normally is reused for stock preparation and dilution of the feed to foam generator 10 at 33.

It will be understood that the foregoing description of a foam forming system is general in nature and included for purposes of illustration and completeness. My invention is not limited to a specific arrangement; on the contrary, it resides in a method for the treatment of the fiuid foam to enhance the physical properties of the resulting sheet and may be carried out with other apparatus embodying the same or similar principles.

FIGS. 2 to 6 illustrate specific examples of nozzles that may be employed in accordance with my invention to apply controlled shear forces to the foam in its fluid state. The nozzle shown in FIGS. 2 and 3 includes a rectangular opening 40 formed by surface 41, ends 42, 43 and insert 44. The amount of shear for a given flow can be adjusted by the thickness of insert 44 which is removably attached as by means of bolts 46. It is preferred that insert 44 include beveled portion 45 for improved fluid flow. Attachment of the nozzle to foam generator 10 is by means of bolts 47, for example.

In the nozzle arrangement shown in FIGS. 4 and 5, the shear forces are controlled by moving plate 52 towards 4 or away from plate 53. Bolts slide within slots 54 and may be tightened to maintain plate 52 in a desired position.

A third nozzle configuration is illustrated in FIG. 6. In this modification plate 56 pivots about joint 58 to adjust the size of opening 60 and thereby the shear stresses applied to the foam. Plate 56 is maintained in position by adjustable locking piston 61. As shown, this nozzle is adapted for horizontal foam formation on screen 62 as it moves around breast roll 64.

The operation of my invention will now be described with particular reference to FIGS. 7 to 9. FIG. 7 is a partial sectional view of foam generator 10 and nozzle 12 which is generally of the type illustrated in more detail by FIGS. 2 and 3. As the foam 13 fiows from the foam generator 10 into the nozzle 12, fibers 66 and bubbles 6.7 are in generally random distribution forming the foam as is shown in greater detail by FIG. 8. The shear forces acting on the foam in nozzle 12 tend to generate slippage as indicated in somewhat exaggerated form by displacement line 68 in FIG. 9. This slippage results from relative motion between foam segments throughout section 13A as well as at the interface of sections 13a and 13b, and the fibers subjected to slippage become aligned by this relative movement as indicated schematically in FIG. 9.

In operation of my invention to produce a sheet having significant orientation, it is necessary to start with a foam wherein the fibers span more than one bubble, and fibers of wood pulp or synthetics of a length of at least about 1 inch are suitable. The foam preferably has a consistency (wt. of fibers/wt. of stock in the range of from about 1% to about 3% and feed rate of about 8 gal/min. to 40 gal/min. per foot of generator width depending on the type and weight paper being produced. Under such conditions the lack of relative motion within the liquid membranes surrounding the air bubbles apparently results in the fibers being securely retained. So held by the foam, the fibers, once well dispersed in the foam, are not free to move independently of the membranes and do not become flocculated or entangled. Accordingly, if a foam with fibers randomly dispersed in it is subjected to plug flow, where, in effect, all bubbles move together, the random fiber orientation is maintained during movement, and also during any simple subsequent breaking of the foam to form a product. However, by providing for a controlled shear effect in the foam body during the transport of the foam body, such that a yield flow condition exists, a desired zone of bubbles may be moved relative to another or other zones. This relative motion caused between bubbles or zones of bubbles will orient the fibers, I have found, to a marked extent, such that the control is reflected in the strength characteristics of a formed sheet or web.

Since fiber alignment in the foam fiber dispersion begins when yield flow starts, it would be convenient for design and operation control purposes to know when yield flow occurs. The relationship existing between yield and plug flow within a particular system can be correlated in general terms. Once this relationship is established, it shows how parameters can be manipulated to obtain the type and extent of flow desired. I believe that a transition from totally plug flow to commencement of yield flow can be correlated by an expression of this form:

{um (W6) ig W +W CdL The terms in this expression are defined as follows using any set of consistent units as one pleases:

W =Weight of liquid in weight units per unit (volumetric or gravimetn'c) of foam W =Weight of gas or air in weight units per unit of foam A=Cross-sectional area perpendicular to direction of flow in (linear units) C=Wettet perimeter around cross-sectional area A above in linear units.

dP/dL=Pressure gradient in force units/unit area per unit length in direction of flow n and K are constants to be determined experimentally.

For most practical cases, one can assume that 1 generally are easily determined, and they are defined as follows:

1L=Distance in linear units between two points along a dimensionally uniform section of conduit.

1P=Pressure difference in force unit/unit area across Plug flow occurs in a fiber foam system when the value of the left hand side of the above equation is less than K. K being a characteristic constant for foams made with fluids of the same chemical composition. Yield flow occurs when the value of the left hand side of the equation is equal to or greater than K.

The constants n and K can be determined by experimental observations. For example, a relatively simple method which yields results of sufiicient accuracy involves visually noting the flow transition in a transparent conduit where the conduit is of uniform, known cross-section, A, the wetted perimeter, C, and the distance between two pressure tap points may be easily measured. A suitable diiferential measuring device may be placed across the two pressure taps to measure pressure differences. Then a foam of known gas to liquid ratio is caused to slowly flow through the conduit. The flow rate is gradually increased until transition from totally plug to commencement of yield flow is visually observed. At this time the pressure differential is noted, and all variables in the above equation are known except for n and K. Repetition of this exercise with the same gas and liquid at a different ratio yields a second set of data. With the two equations, 11 and K may be readily determined.

EXAMPLE This example demonstrates the effect of varying the shear stress on the foam by changing nozzle dimensions. In Runs B and C an arrangement generally like that shown in FIG. 2 was used while Run A utilized similar apparatus except that the foam was directed from the generator onto horizontal twin screens. In all cases the consistency of the feed stock to the generator was 1%. The fiber furnish was unrefined stock consisting of 50% Kraft and 50% sulfite. The surfactant was Alipal AB436 (ammonium salt of a sulfated linear primary alcohol ethoxy1ate-a trademark of GAF Corp.) and added to the liquid feed in the amount of about 0.25% by volume as received. In Runs A and B the machine speed was 90 f.p.m. while Run C was performed at a machine speed of 120 f.p.m. The following table summarizes the results of these runs:

Thus I believe it has been demonstrated that the selection of particular nozzle dimensions can be made to produce a desired MD/CD ratio or, correspondingly, a desired degree of fiber orientation for a given machine speed. In effect, the proportion of foam experiencing yield flow is controlled thereby producing the desired orientation.

It will be recognized by those skilled in this art that there are other methods of applying shear forces to the foam to generate a yield flow region. For example, wires 14 and 16 may be speeded up relative to the velocity of the depositing foam. It is therefore, not intended that the invention be limited to a particular apparatus.

In the case where the foam is oriented to a greater degree than is desired or in a mode which is considered more diflicult to correct, it is preferred that it be mixed to a substantially randomly oriented state and then treated as above in accordance with my invention. This mixing should take place while the foam is still fluid and may be accomplished in a number of ways known to those skilled in this art. For example, impellers which do not break the foam may be used for mixing, the foam may be passed through a turbulator section, or two or more foam streams may be combined to cause mixing.

The range of physical property control obtainable through the use of my invention is, of course, dependent upon a wide variety of factors. Among these are fiber composition, fiber length, sheet thickness, and orientation of the fibers as produced by the foam generator. However, using pulp or other natural or synthetic fibers of nominal lengths in the range of from about to to form sheets of generally uniform thickness in the range of from 5 to 50 lbs./ 3000 ft. and starting with a randomly oriented foam, I have been able to control the MD/CD strength ratio with results ranging from 1:1 to 9:1. The former, of course, was obtained by maintaining nearly plug flow conditions while the latter resulted from the use of a narrower nozzle channel under yield flow conditions. Strength ratios in between often are produced in the yield flow region where there is a two-part flow wherein that part of the flow furthest from the walls moves in the plug -fiow regime, and that part closer to the walls moves in the yield flow regime. In this case, a thin layer of bubbles adjacent the walls moves at the lowest velocity (speed relative to the Wall), and successive lay-' ers from the walls move at progessively higher velocities. The plug in the center, as a whole, moves at the highest velocity. While this may occur in varying degrees in other arrangements, in accordance with my invention it is controlled by subjecting the foam to the desired degree of shear forces.

It will be understood by those skilled in this art that the particular fiber or foam composition is not critical to my invention. On the contrary, I contemplate its use with a wide variety of pulps and synthetic fibers such as Kraft, sulfite, groundwood, hemp, cotton, chemically modified cellulosic fibers, nylons, olefins, polyesters, acetates, triacetates, metallic, carbon, carbide, polyurethanes, vinyl, vinyon, rayons, acrylic, glass, bagasse, or combinations of these in any proportion, for example. Any of the foaming agents may be utilized, such as the commonly used anionic, non-ionic, and cationic surfactants, soaps, modified proteins, organophospho-acids and fiuorochemical types. Specific examples of surfactants are: sodium or ammonium salts of a sulfated alkylphenoxypoly (ethyleneoxy) ethanol; nonylphenoxypoly (ethyleneoxy) ethanols; iso-octyl phenyl polyethoxy ethanols; polyoxyethylene sorbitan monopalniitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan mono-laurate, polyoxyethylene sorbitan mono-oleate, polyoxyethylene sorbitan tristearate, polyoxyethylene sorbitan tri-oleate. Cationic surfactants are substantive or tend to be completely absorbed on cellulosic fibers and may reduce bond strength in the finished dry web. On the other hand, cationic surfactants are sometimes preferred when it is desired that retention of the detergent impart special properties such as bacteriostatic to the finished product. For similar purposes, surfactants containing Zwitterion or quaternary ammonium groups may be useful. Or the foam may be formed by other known methods such as Denver Equipment Co.s Froth Flotation Cell or beater-type agitators.

I claim:

1. In a method of foam-forming a paper web having a basis weight in the range of from about 5 to about 50 pounds per 3000 sq. ft. comprising the steps of, forming a random mixture of cellulose fibers in a fluid foam comprising bubbles formed by air within an aqueous membrane wherein said fibers generally span more than one bubble, depositing said mixture from an opening onto a carrier thereby forming layers of said bubbles and fibers, and removing said fluids,

the improvement wherein the degree of alignment of said fibers in the paper web in the machine direction is controlled by varying the wetted perimeter of said opening to produce yield flow and relative movement between said bubble layers within said fiuid foam, said orientation being increased as the wetted perimeter is decreased.

2. The process of claim 1 wherein said foam is formed by agitation of an aqueous dispersion of fibers with a surfactant.

References Cited UNITED STATES PATENTS 3,506,538 4/1970 Friedberg et a1 162341 3,716,449 2/1973 Gatward et a1. 162l()1 3,617,437 11/1971 Bagg et a1 162102 OTHER REFERENCES McCabe, Warren L., et al., Unit Operations of Chemical Engineering, McGraw-Hill Book Co, 1967, New

10 York, pp. 90-94.

S. LEON BASHORE, Primary Examiner W. F. SMITH, Assistant Examiner US. Cl. X.R. 162203 

